This invention relates generally to motor controls and, more particularly, to a control system and method of control for a switched reluctance motor operating as a power generator.
Switched reluctance motors conventionally have multiple poles or teeth on both stator and rotor, i.e., they are doubly salient. There are phase windings on the stator but no windings on the rotor. Each pair of diametrically opposite stator poles is connected in series to form one phase of a multi-phase switched reluctance motor. Torque is produced by switching current into each of the phase windings in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of the phase rotate past the aligned position. Otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be applied to develop torque in either direction. These pulses are generated by a converter using current switching elements such as thyristors or transistors.
In operation, each time a phase of the switched reluctance motor is switched on by closing a switch in a converter, current flows in the stator winding of that phase providing energy from a direct current (DC) supply to the motor. The energy drawn from the supply is converted partly into mechanical energy by causing the rotor to rotate toward a minimum reluctance configuration and partly in stored energy associated with the magnetic field. After the switch is opened, part of the stored magnetic energy is converted to mechanical output and part of the energy is returned to the DC source.
U.S. Pat. No. 4,707,650 describes a control system for a switched reluctance motor employing a programmable, closed loop, four quadrant control system incorporating feedback control, angle control and current control. The feedback control incorporates a speed feedback loop and/or a torque feedback loop. The angle control digitally synchronizes stator phase current pulses with rotor position, and the current control acts as a chopping or bang-bang controller to limit the magnitude of the stator phase current pulses. The magnitude and turn-on and turn-off angles of the stator current pulses for each phase, in feedback mode, are controlled so as to provide smooth operation and full torque and speed range with optimum performance in all four quadrants of motor operation, i.e., forward motoring, forward braking, reverse motoring and reverse braking.
The switched reluctance motor can be utilized as a generator in the braking mode. When operated as a generator, the motor produces current rather than voltage. Braking torque is produced when winding current continues to flow after a rotor pole has passed alignment with an associated stator pole. Because the switched reluctance motor has no rotor excitation, it is necessary to first draw electric power from a DC bus in order to cause current to begin flowing in windings of the motor. Current can be initiated in the windings either prior to alignment of a rotor pole and associated stator pole or after alignment has occurred. In general, very little torque will be produced by currents which exist when a corresponding rotor pole is adjacent or close to either side of a stator pole. Once the rotor pole passes alignment or continues into the negative torque region, the winding current will build faster than in the motoring region because the inductive term which establishes the voltage across the motor winding becomes negative. While some DC current will still be drawn from the associated DC bus while generating torque is being produced, DC current will be delivered to the bus when the switches actuated to start current into the winding are turned off and force the winding current to commutate into the associated flyback diodes. The net DC current is the sum of all the current from all of the phases of a multi-phase motor and it is this net DC current which is desired to be regulated when the reluctance motor is operated as a generator.
In some applications, a switched reluctance motor can be operated to function as a motor during start-up of a system and thereafter act as a generator after the system has become started. For example, if the reluctance motor is applied to act as a starter for a gas turbine engine, the motor may be called upon to bring the gas turbine engine up to its self-sustaining speed, and thereafter to act as a generator throughout the gas turbine's power producing speed range. The desired method of control in the generating mode is that of a voltage regulator since electrical loads can be supplied by the DC link voltage.